Caitlin nic Raighne has created a website with instructions on how to make a geometric tunic. The instructions include diagrams, fabric selections and sewing directions.
From the introduction:
The geometric tunic was the garment in general use for everyday wear up to about 1250. It is loose, unshaped and simple to make. Men's and women's garments differ only in the length and the amount of fullness in the hem: tunics are about knee-length, often slit at the front and back or sides. Dresses are always ground length. Sometimes they can be pouched over the girdle for working.